The present invention relates to the control of rod type pumping units used in the production of crude oil. In a rod type pumping unit, a reciprocating pump is located at the bottom of the well and actuated by a string of pump rods extending up the well to the surface. At the surface a beam pumping unit reciprocates the rods to actuate the pump. The pump has a stationary valve at the bottom and a traveling valve in the piston with the stationary valve opening on the upstroke of the piston to allow fluid from the formation to flow into the pump barrel. On a succeeding downstroke the stationary valve closes while the traveling valve in the piston opens to allow the fluid in the pump barrel to flow through to the top of the piston. On the succeeding upstroke the fluid on top of the piston is lifted to the surface of the well. The beam pumping unit at the surface is normally driven by an electric motor with provisions being made for controlling the starting and stopping of the motor depending upon the fluid level in the well. As explained in the above patents when there is insufficient fluid in the well, the pump barrel is not completely filled on the upstroke and on the succeeding downstroke the rod string and piston fall free due to the weight of the rod string and the column of oil above the piston until it contacts the oil. By falling free, the piston will build up sufficient speed to cause considerable shock loads to occur in the pumping unit when the piston contacts the fluid in the pump barrel. The mechanical shock results in considerable pounding and vibration of the pump unit and causes considerable damage. Thus, many attempts have been made to determine when the pump barrel does not completely fill with fluid so that the pump unit can be shut down to allow additional fluid to drain in to the reservoir. The condition of the pump barrel not filling with fluid is normally referred to as pumped-off condition while the complete filling of the pump barrel is referred to as a pumping or full-pump condition.
The above patents describe a method in which the load on the rod string and the displacement of the rod string are used to determine the energy input to the rod string. The energy input is determined by computing the area of the dynamometer card for the well. The energy input is then compared with the energy input under full-pump conditions to determine when the well has been pumped-off in order to control the pumping unit. While this approach is successful, it requires the accumulation of the load and displacement data for a complete cycle of the pump before the energy input to the rod string can be determined. Further, the data must either be plotted to form a pump dynamometer card or must be manipulated in a computer to determine the area of the dynamometer card. This means that the determination cannot be made in real time.